Streamサンプル
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import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
static class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
public String toString() {
return String.format("name=%s,age=%d", this.name, this.age);
}
}
public static List<Person> getData() {
List<Person> personList = new ArrayList<>();
personList.add(new Person("Alice", 25));
personList.add(new Person("Bob", 30));
personList.add(new Person("Charlie", 35));
personList.add(new Person("Dave", 35));
return personList;
}
public static void main(String[] args) {
var personList = getData();
// nameを取り出し
List<String> upperCaseNames = personList.stream()
.map(person -> person.getName().toUpperCase())
.collect(Collectors.toList());
System.out.println("* map result1");
System.out.println(String.join(",", upperCaseNames));
// nameを取り出し
List<String> names = personList.stream()
.map(Person::getName)
.collect(Collectors.toList());
System.out.println("* map result2");
System.out.println(String.join(",", names));
// Filter処理
List<Person> filteredList = personList.stream()
.filter(person -> person.getAge() >= 30)
.collect(Collectors.toList());
System.out.println("* filter result");
filteredList.forEach(it -> {
System.out.println(it);
});
// Group by 処理
Map<Integer, Long> ageCount = personList.stream()
.collect(Collectors.groupingBy(Person::getAge, Collectors.counting()));
System.out.println("* group-by result");
ageCount.keySet().stream().forEach(key -> {
System.out.printf("%s = %s\n", key, ageCount.get(key));
});
}
}
Java
INFO